1. Field of the Invention
The present invention generally relates to a flexible flat conductor with at least two electrically conductive layers, which are at least partially surrounded by an electrically insulating cover, the electrically conductive layers being insulated from one another by at least one dielectric layer arranged between them.
Furthermore, the invention relates to a power supply unit which includes such a flexible flat conductor.
2. Description of the Related Art
Power supplies and chargers in the low power range are implemented nowadays as a switched mode power supply unit to meet requirements in respect of the wide input voltage range and low losses. An embodiment of this device which is in widespread use takes the form of a plug-in power supply unit 1, wherein an electronic circuit for power conversion is accommodated in a housing located in the immediate vicinity of the mains plug, as shown in FIG. 1. A plurality of such devices is used for charging portable devices such as mobile phones, PDAs, CD/DVD/MD/MP3 playback devices and the like. Portability is largely a question of the size of the charger, its weight and ease of transport. The connection to the consumer (not shown in the figure) is normally effected by means of an output plug 2 and a two-pole output line 3, which is a round line or twin line, as is shown in FIG. 1.
In addition, it is known to use in such power supply units flat cables equipped with a wind-up device. An example of such an arrangement is shown e.g. in JP 2001/128350 and WO 01/21521 A1. Such arrangements allow for a particularly space-saving and orderly accommodation of the cable during transport.
Power conversion is normally achieved nowadays with a flyback converter, which is preferred on account of its comparatively uncomplicated circuitry in this power range. If the energy transmission takes place by means of primary control, as shown in DE 100 18 229 A1, only a diode for rectification and an LC filter for filtering the output voltage are provided on the secondary side. A circuit diagram of such a known output-side circuit is shown in FIG. 2. Whereas a ceramic capacitor is normally employed for the capacitor C2 in FIG. 2 an electrolytic capacitor is usually chosen for the capacitor C1 to meet the requirements of low equivalent series resistance at minimal cost. Typical characteristic values for the components shown in FIG. 2 are:                C1: 22 μF . . . 470 μF        L: 1 μH . . . 100 μH        C2: 10 pF . . . 10 μF        
As is shown in FIG. 3, this arrangement is usually followed by a current-compensated choke L3′ with terminating filter capacitor C3 in order to suppress common mode interference. As traditional discrete components the filter arrangements shown in FIGS. 2 and 3 occupy a considerable amount of space in the plug-in power supply unit and thus hinder further miniaturization of the power supply unit. Additionally, high-frequency interference may be coupled in via the output line. This usually necessitates an additional input filter within the consumer, thus resulting in an increase in the size, weight and cost of the consumer.
Finally, the practice of fabricating filter structures integrated with a flexible flat conductor in order to make them as simple, cheap and compact as possible is known. A flexible flat cable with electronic components integrated therein is known from Japanese Laying Open Publication JP 06-139831 A. Various conductive structures surrounded by an electrical insulation are here insulated from one another by a further dielectric layer so that a capacitor is formed. By means of a meandrous patterning of the conductor levels, an inductance can be realized after a subsequent folding process wherein the individual meanders are superimposed in the shape of a concertina folding in the third dimension. Here the combination of capacitance and inductance provides an integrated filter.
However, this solution is disadvantageous in that, in order to implement the inductances needed for a filter structure, the flexible flat conductor must be folded many times in a particular way, resulting not only in an increased outlay during production but also to more space being needed. In addition, as a consequence of the necessary folding of the flexible flat conductor according to JP 06-139831 A only certain regions of the flexible flat conductor can be utilized for the integrated filter structure, thus leaving long stretches of the cable unused.